Recent increase in packing densities of LSI circuits has revealed limitations of resolutions to which traditional resists of monolayer type can reach. So a method of making a resist have a multilayer structure, and not a single layer structure, and forming fine patterns which have great coating thickness, and profiles of high aspect ratios besides, has been proposed. More specifically, a thick coating of organic high polymer is formed as the first layer, and thereon a thin resist material layer is formed as the second layer. Thereafter, the resist material of the second layer is irradiated with high-energy beams, and then developed. By using the thus obtained patterns as a mask, the organic high polymer of the first layer is etched anisotropically by oxygen plasma etching (O2-RIE) to form patterns whose profiles are highly rectangular (See Lin, Solid State Technology, vol. 24, p. 73 (1981)).
This double-layer resist method has an advantage that the second resist layer can be reduced in thickness, and thereby enabling achievement of a high resolution, a high aspect ratio and a great focus depth.
In this case, the second resist layer is required to have high resistance to O2-RIE, so that silicon-containing polymers are generally used for the second resist layer. Many attempts to use vinyl polymers having in their side chains silicon-containing acid-decomposable groups have been made, especially on the grounds that such vinyl polymers can ensure a high degree of freedom in molecular design, starting materials thereof can be easily available and they can be synthesized with ease. For instance, they are disclosed in Patent Document 1 (JP-B-7-99435), Patent Document 2 (U.S. Pat. No. 5,856,071), Patent Document 3 (WO 02/73308A1), Patent Document 4 (JP-A-2001-305737) and Patent Document 5 (JP-A-2002-256033)
Although those double-layer resist methods were applied with the intention of, e.g., forming fine patterns in the vicinity of limiting resolution of 0.14 μm or below by use of ArF laser as exposure light source, they were unable to deliver satisfactory resist performances, including high resolution, good mask linearity of critical dimension (CD), scum free, reduction in thinning of resist film and reduction in SEM shrink (shrink occurring at the time of observation under a scanning electron microscope).    [Patent Document 1] JP-B-7-99435    [Patent Document 2] U.S. Pat. No. 5,856,071    [Patent Document 3] WO 02/73308A1    [Patent Document 4] JP-A-2001-305737    [Patent Document 5] JP-A-2002-256033